In audio engineering it is very difficult to produce a single sound driver to operate over the full audio frequency range of 20 Hz to 20,000 Hz (equivalent to wavelengths from 17 m to 17 mm respectively). For low frequency sound, the sound driver itself has to be physically large to generate the low frequency sound pressure with the required amplitude. As the sound frequency increases, the sound driver will tend to exhibit increasingly irregular radiation patterns as its dimensions become comparable to and larger than the radiated wavelengths of sound being generated. Accordingly, a smaller sound driver is needed to radiate a more uniform pattern of sound at these higher audio frequencies. As a consequence, to achieve operation over the full audio bandwidth, typically a plurality of sound drivers is commonly used with larger transducers for lower frequency ranges and progressively smaller sound drivers for high frequency ranges.
It is the general practice to arrange these sound drivers in a vertical array with the higher frequency drivers located at the top of the array. Frequency dependent electronic networks, generally referred to as a “crossover networks” are then used to direct bands of frequencies to the appropriate sound driver for that particular frequency band. An important design goal generally for sound drivers and sound reproduction systems is maintenance of the directional characteristics of the generated sound field over the audio frequency range. This is usually assessed in vertical and horizontal planes which intersect the design axis of the loudspeaker system, that is, the on-axis point at which the frequency response is measured.
One common measure of the directional characteristics of the sound field is termed the “beam angle”. This quantity is defined at a given frequency as the angle between the off-axis points that are 6 dB lower than the on-axis sound pressure level (SPL). The design goal is then to maintain the beam angle substantially constant over the audio frequency range of the sound reproduction system. As a consequence, graphs of beam angle versus frequency are often included as part of the data pertaining to a sound reproduction systems as an indication of the directional characteristics of the system.
In the case of multiple sound drivers, a further design goal is to ensure that there is a uniform SPL between the outputs of the sound drivers to create a uniform response across the frequency range. Accordingly, for a sound reproduction system involving two sound drivers at the relevant crossover frequency the output of the two sound drivers is adjusted to radiate equally. Referring now to FIG. 1, there is shown an idealised case of two identical vertically arranged sound drivers 100, 110 of equal strength functioning as omni-directional sources of sound radiation corresponding to this arrangement. While the on-axis frequency response can be adjusted to give a uniform sound output from one sound driver to the next due to the equal path lengths (e.g. 120) from the sound drivers 100, 110, the vertical polar response is affected by dips and nulls as the off-axis path length difference becomes multiples of one half wavelength (e.g. 130) resulting in cancellation occurring.
This effect is depicted in FIG. 2A which shows the polar graph 200 for the arrangement illustrated in FIG. 1 and the null in the generated sound field at the off-axis angle where the path difference is one half wavelength. The effect also occurs at higher frequencies where the difference would be three half wavelengths, five half wavelengths, and so on. FIG. 2B shows an acoustically measured polar graph 210 of two stacked horns at the crossover frequency of 1510 Hz, in which the off-axis nulls are blurred by the finite size of the horns but the beam angle of the main central lobe is smaller than either of the two horns at that frequency. Additional off-axis nulls are seen in this example, as the centres of the horns are two wavelengths apart at the crossover frequency. This vertical off axis cancellation whose directional characteristics will vary as a function of wavelength degrades the design goal of achieving a uniform vertical beam angle over the frequency range of the sound reproduction system.
Referring now to FIGS. 3A and 3B there are shown side and top diagrams of a typical acoustic horn 300 as is known in the art. An acoustic horn 300 is a structure which utilises continuous outwardly flaring rigid walls 330 to provide an expanding passage for acoustic energy originating from a sound driver 310 located at a throat entrance 320 and radiating towards a mouth exit 340. The throat section 321 of acoustic horn 300 extends away from the throat entrance 320 into a feeder section 322 that is generally rectangular in transverse cross-sectional shape. The feeder section 322 has an expanding transverse area formed by a first pair of walls 331 that diverge outwardly from each other, and a second pair of walls 332 that are substantially parallel and joined to the first pair 331. As such, the configuration of acoustic horn 300 defines both vertical and horizontal directions with respect to the acoustic horn as depicted by the V and H arrows depicted in FIGS. 3A and 3B respectively.
The mouth exit 340 of the horn has a rectangular configuration and is formed by a bell section 323 having walls diverging outwardly from the end of the feeder section consisting of a first pair of diverging walls 333, and a second pair of diverging walls 334 that join with the first pair of walls 333 of the bell section 323 along the edges to form an integral unit. The walls 333, 334 of the bell section 323 may be flared outwardly an additional amount at a transverse plane immediately adjacent to the mouth to provide improved control of the radiation of acoustic energy. It is understood that feeder section 322 may be quite short in some implementations and that the mouth exit 340 may be square depending on the required characteristics of the horn.
The divergence angle between the first pair of walls 331 and between the second pair of walls 334 of the bell section 323 generally determines the dispersion angle of the acoustical energy. A further refinement of acoustic horn 300 is known as a Constant Directivity (CD) horn where the horn geometry is optimised to have a predetermined area of coverage typically defined by the coverage angle in a horizontal plane by the coverage angle in a vertical plane (e.g. 90° by 40° or 60° by 40°).
Referring now to FIG. 4A, there is shown a figurative depiction of the side view of the simulated sound field 450 produced by an exemplar of a constant directivity acoustic horn 400 having a centreline 480 similar to the type depicted in FIGS. 3A and 3B assuming a sound frequency of 4000 Hz. Acoustic horn 400 includes a throat 421 where the sound driver (not shown) is located and a mouth 440. The simulated sound field 450 of acoustic horn 400 represents a map of the acoustic phase of the sound energy within the horn 400 where darker regions 410 indicate areas of positive phase and lighter regions 415 represent areas of negative phase. Acoustic phase has been adopted in this depiction as it provides a clearer delineation of the wave fronts of the sound propagating through the acoustic horn. White lines 430 have been inserted to indicate the spherical wavefronts of the sound field 450. FIG. 4B shows the polar graph obtained at a nominal frequency of 4000 Hz, and FIG. 4C shows the vertical and horizontal beam angles over the principal audible range between 500 Hz to 8 kHz, indicating the base line performance of a standard acoustic horn.
One attempt to address the problem with standard arrays of sound drivers referred to above is to arrange the sound drivers in a concentric or collinear arrangement. The problem then shifts to that of devising a method for allowing the radiation of sound from the same point in space, or from collinear closely spaced acoustic sources of sound radiation. Digital signal processing techniques have been developed that allow for different time delays to be implemented for collinear closely spaced sources so that the generated sound field can effectively come from the same point.
One such arrangement involves installing a smaller acoustic horn in the mouth of a larger acoustic horn and is shown in FIG. 3C. In this arrangement, the smaller horn 391 obstructs the mouth of the larger horn 392 and in effect turns the larger horn 392 into a ring radiator of acoustic energy. Considering a cross section in the vertical plane, the smaller horn 391 blocks sound radiation from the centre of the larger horn 392, thereby effectively creating two sources, upper and lower, similar in respect to that illustrated in FIG. 1. This then again results in path length differences at off-axis angles and the vertical polar response is once again affected by dips and nulls as the off axis path length difference becomes multiples of one half a wavelength and cancellation occurs as shown in FIG. 2A. A similar observation applies'to a sectional view taken in the horizontal plane. In effect the smaller horn 391 ‘shadows’ the central part of the mouth of the larger horn 392 as shown in FIG. 3C.
A further arrangement that has been proposed is the use of a single source such as an acoustic horn that is driven by more than one driver and accordingly there have been a number of attempts to combine multiple sound driver outputs into a unified acoustic horn geometry of the type depicted in FIGS. 3A and 3B. One configuration involves the vertical stacking of separate horn elements within the shared side walls of the acoustic horn. However, this configuration also has the disadvantage of once again effectively creating two vertically space acoustic sources with a resultant polar radiation graph as depicted in FIG. 2.
Another potential configuration is to introduce sound from multiple sound drivers, into the acoustic horn by an aperture, functioning as a sound transfer or interface region. In this arrangement, the horn will typically be driven by a first sound driver located on axis at the throat entrance of the acoustic horn that generates sound in a first frequency range and then include one or more further sound drivers generating sound in other frequency ranges, this sound then being introduced into the horn via an aperture or series of apertures located in the walls of the acoustic horn.
The introduction of sound from one or more additional sound drivers into the acoustic horn via the interface region results in a modification of the sound field generated by the central sound driver driving the acoustic horn which adversely affects directional characteristics of the resultant combined sound field radiated by the acoustic horn. This effect primarily results from the interaction of the sound field of the first central sound driver with the interface region which is necessary to introduce into the acoustic horn the sound from the second sound driver.
This effect is depicted in FIGS. 5A to 5D where the applicant has conducted a number of simulations and measurements directed to a variety of different aperture or interface region arrangements 520 located in the walls of the acoustic horn. In each case A to D it is found that there are substantial variations in the beam angle versus frequency. It is postulated that the wavefronts of the sound field from the first sound driver located at the throat of the horn are distorted as a result of the aperture or apertures responsible for introducing sound from a second driver. It can be seen that while the initial wavefront 500 is spherical, the wavefront is then modified by the interface region and remains modified as it propagates towards the mouth resulting in a distorted and sub-optimal wavefront 510.
As would be appreciated by those of ordinary skill in the art spherical wavefronts area necessary but not sufficient condition of a monotonic polar response i.e. the maximum SPL occurs on-axis, and progressively reduces the further the observation point moves off-axis. The shape of the polar response and hence the beam angle between the −6 dB points is determined by the smoothness or otherwise of the variation of SPL from a maximum on-axis to a lower level at right angles to the axis of the horn.
There is therefore a need for an acoustic horn arrangement which is capable of being driven by multiple sound drivers to increase the frequency range of the acoustic horn while still substantially maintaining the horn's directional characteristics.